1. Field of the Invention
This invention is related to wellbore milling processes, milling systems, milling tools and whipstocks.
2. Description of Related Art
Milling tools are used to cut out windows or pockets from a tubular, e.g. for directional drilling and sidetracking; and to remove materials downhole in a well bore, such as pipe, casing, casing liners, tubing, or jammed tools. The prior art discloses various types of milling or cutting tools provided for cutting or milling existing pipe or casing previously installed in a well. These tools have cutting blades or surfaces and are lowered into the well or casing and then rotated in a cutting operation. With certain tools, a suitable drilling fluid is pumped down a central bore of a tool for discharge adjacent or beneath the cutting blades. An upward flow of the discharged fluid in the annulus outside the tool removes cuttings or chips from the well resulting from the milling operation.
Prior art sidetracking methods employ a variety of wellbore mills, including, but not limited to, well known starting mills and window mills. A whipstock deflects a mill laterally while it is being moved downwardly in a well during rotation of the mill to cut an elongated opening pocket, or window in well casing.
Certain prior art well sidetracking operations which employ a whipstock also employ a variety of different mills and milling systems used in a certain sequence. This sequence of operation may require a plurality of "trips" into the wellbore. For example, in certain multi-trip operations, an anchor, slip mechanism, or an anchor-packer is set in a wellbore at a desired location. A whipstock-mill combination tool is then run into the wellbore by first properly orienting a stinger at the bottom of the tool with respect to a concave face of the tool's whipstock. Typically a starting mill or a window mill is releasably secured at the top of the whipstock, e.g. with a shearable member, e.g. a shearable screw or a setting stud and nut connected to a pilot lug on the whipstock. This setting stud bears the entire load of whatever is connected beneath the lowermost mill. The tool is then lowered into the wellbore oriented and anchored. Putting weight down on the tool then shears the setting stud, freeing the lowermost mill, e.g. a window mill or a starting mill from the tool. The mill is diverted into the casing and the casing is milled in some cases as the pilot lug is milled off. The mill moves downwardly while contacting the concave portion and cuts an initial window in the casing. If a starting mill is the lowermost mill, it is then removed from the wellbore. A window mill, e.g. on a flexible joint of drill pipe, is lowered into the wellbore and rotated to mill down from the initial window formed by the starting mill. Then additional mills may be used behind the window mill to lengthen and/or finish the casing window if desired.
There has long been a need for efficient and effective wellbore milling methods and systems in which a significant weight can be imposed on the system without inadvertent shearing of a shear stud connecting a mill to a whipstock, particularly in wellbore sidetracking procedures and the production of multiple lateral wellbores.